Final answer:
Macbeth's words reflect his guilt and foretell his existential crisis after murdering King Duncan, indicating that his quest for power has left him with a life stripped of meaning and honor.
Step-by-step explanation:
When Macbeth says, "Had I but died an hour before this chance I had lived a blessed time; for, from this instant, there's nothing serious in mortality," he is unconsciously describing his own sense of guilt and despair after having murdered King Duncan. By wishing to have died before the murder took place, Macbeth is expressing a deep regret for his actions that have now rendered life meaningless and full of anguish. It's an acknowledgment that everything he valued and considered serious or meaningful in life has been overshadowed by his grievous act.
This regret and nihilistic outlook foreshadow the mental turmoil that will plague Macbeth for the rest of the play. His rise to power following Duncan's death brings him no satisfaction but instead leaves him feeling haunted and paranoid, ultimately leading to his downfall. Macbeth's lamentation reflects a classic theme in literature: the idea that ill-gotten gains result in spiritual and existential emptiness, and sometimes, in the pursuit of power, one can lose their humanity.